Researchers using GPS data to study tides

Pasadena, US: GPS technology is helping researchers at California Institute of Technology (Caltech) find their way to a more complete understanding of Earth’s interior structure. Up until now, the best way to explore Earth’s internal structures—to measure geological properties such as density and elasticity—has been through seismology and laboratory experiments. “However, it is difficult using seismology alone to separate the effects that variations in density have from those associated with variations in elastic properties,” said Mark Simons, professor of geophysics at Caltech’s Seismological Laboratory, part of the Division of Geological and Planetary Sciences.

Now Simons and Takeo Ito, visiting associate at the Seismological Laboratory and assistant professor of earth and planetary dynamics at Nagoya University in Japan, are using data from GPS satellite systems in an entirely new way. They are using data to measure the solid Earth’s response to the movements of ocean tides—which place a large stress on Earth’s surface—and to estimate separately the effects of Earth’s density and the properties controlling response when a force is applied to it (known as elastic moduli). Their work has been already published in Science Express.

By using measurements of Earth’s movement taken from high-precision, continuously recording permanent GPS receivers installed across the western US by the Plate Boundary Observatory (PBO), the researchers were able to observe tide-induced displacements—or movements of Earth’s surface—of as little as one millimetre. PBO is a component of EarthScope, a programme that seeks to understand the processes controlling earthquakes and volcanoes by exploring the structure and evolution of the North American continent.

The team focused on understanding the properties of the asthenosphere, a layer of weak and viscous upper mantle that lies below Earth’s crust, and used those measurements to build one-dimensional models of Earth’s response to the diurnal tides in the western United States.

“The asthenosphere plays an important role in plate tectonics, as it lies directly under the plates,” explains Ito. “The results of our study give us a better understanding of the asthenosphere, which in turn can help us understand how the plates move,” said Simons.

The models provided a look at the variations in density from Earth’s surface down to a depth of about 400 kilometres. Now that the researchers know it is possible to use GPS to derive measurements of internal Earth structures, they anticipate several new directions for this research.

“We hope to extend the observations to be global in scope, which may require temporary deployments of GPS in important areas that are typically tectonically bland—in other words, devoid of significant earthquakes and volcanoes—and thus do not have existing dense continuous GPS arrays already in place,” said Simons. Next steps may also include going beyond the current one-dimensional depth-dependent models to build 3D models and combining the GPS approach with more conventional seismic approaches.